The insulated gate bipolar transistor (IGBT) is a semiconductor device that combines many of the desirable properties of a field-effect transistor (FET) with those of a bipolar junction transistor (BJT). An exemplary conventional IGBT device 10 is shown in FIG. 1. The conventional IGBT device shown in FIG. 1 represents a single IGBT cell that includes an IGBT stack 12, a collector contact 14, a gate contact 16, and an emitter contact 18. The IGBT stack 12 includes an injector region 20 adjacent to the collector contact 14, a drift region 22 over the injector region 20 and adjacent to the gate contact 16 and the emitter contact 18, and a pair of junction implants 24 in the drift region 22.
Each one of the junction implants 24 is generally formed by an ion implantation process, and includes a base well 26, a source well 28, and an ohmic well 30. Each base well 26 is implanted in the surface of the drift region 22 opposite the injector region 20, and extends down towards the injector region 20 along a lateral edge 32 of the IGBT stack 12. The source well 28 and the ohmic well 30 are formed in a shallow portion on the surface of the drift region 22 opposite the injector region 20, and are contained by the base well 26.
A gate oxide layer 34 is positioned on the surface of the drift region 22 opposite the injector region 20, and extends laterally between a portion of the surface of each one of the source wells 28, such that the gate oxide layer 34 partially overlaps and runs between the surface of each source well 28 in the junction implants 24. The gate contact 16 is positioned over the gate oxide layer 34. The emitter contact 18 includes two portions in contact with the surface of the drift region 22 opposite the injector region 20. Each portion of the emitter contact 18 on the surface of the drift region 22 opposite the injector region 20 partially overlaps both the source well 28 and the ohmic well 30 of one of the junction implants 24, respectively, without contacting the gate contact 16 or the gate oxide layer 34.
A first junction J1 between the injector region 20 and the drift region 22, a second junction J2 between each base well 26 and the drift region 22, and a third junction J3 between each source well 28 and each base well 26 are controlled to operate in one of a forward-bias mode of operation or a reverse-bias mode of operation based on the biasing of the conventional IGBT device 10. Accordingly, the flow of current between the collector contact 14 and the emitter contact 18 is controlled.
The conventional IGBT device 10 has three primary modes of operation. When a positive bias is applied to the gate contact 16 and the emitter contact 18, and a negative bias is applied to the collector contact 14, the conventional IGBT device 10 operates in a reverse blocking mode. In the reverse blocking mode of the conventional IGBT device 10, the first junction J1 and the third junction J3 are reverse-biased, while the second junction J2 is forward biased. The reverse-biased junctions J1 and J3 prevent current from flowing from the collector contact 14 to the emitter contact 18. Accordingly, the drift region 22 supports the majority of the voltage across the collector contact 14 and the emitter contact 18.
When a negative bias is applied to the gate contact 16 and the emitter contact 18, and a positive bias is applied to the collector contact 14, the conventional IGBT device 10 operates in a forward blocking mode. In the forward blocking mode of the conventional IGBT device 10, the first junction J1 and the third junction J3 are forward biased, while the second junction J2 is reverse-biased. The reverse-bias of the second junction J2 generates a depletion region, which effectively pinches off the channel of the conventional IGBT device 10 and prevents current from flowing from the collector contact 14 to the emitter contact 18. Accordingly, the drift region 22 supports the majority of the voltage across the collector contact 14 and the emitter contact 18.
When a positive bias is applied to the gate contact 16 and the collector contact 14, and a negative bias is applied to the emitter contact 18, the conventional IGBT device 10 operates in a forward conduction mode of operation. Similar to the forward blocking mode of operation, in the forward conduction mode of operation of the conventional IGBT device 10, the first junction J1 and the third junction J3 are forward-biased, while the second junction J2 is reverse-biased. However, in the forward conduction mode of operation, the positive bias applied to the gate contact 16 generates an inversion channel on the surface of the drift region 22 opposite the injector region, thereby creating a low-resistance path for electrons to flow from the emitter contact 18 through each one of the source wells 28 and each one of the base wells 26 into the drift region 22. As electrons flow into the drift region 22, the potential of the drift region 22 is decreased, thereby placing the first junction J1 in a forward-bias mode of operation. When the first junction J1 becomes forward-biased, holes are allowed to flow from the injector region 20 into the drift region 22. The holes effectively increase the doping concentration of the drift region 22, thereby increasing the conductivity thereof. Accordingly, electrons from the emitter contact 18 may flow more easily through the drift region 22 to the collector contact 14.
The IGBT stack 12 of the conventional IGBT device 10 is Silicon (Si), the advantages and shortcomings of which are well known. In an attempt to further increase the performance of IGBT devices, many have focused their efforts on using wide band-gap materials such as Silicon Carbide (SiC) for the IGBT stack 12. Although promising, conventional IGBT structures such as the one shown in FIG. 1 are generally unsuitable for use with wide band-gap materials such as SiC. Due to inherent limitations in SiC fabrication processes, the carrier mobility and/or carrier concentration in the injector region 20 in a SiC IGBT device may be significantly diminished. Specifically, the conductivity in the injector region 20 will be low in a SiC device due to difficulties in growing high quality P-type epitaxial layers with low defect density. Further, due to damage in the drift region 22 caused by the ion implantation of the junction implants 24, the lifetime of carriers in the area directly below each junction implant 24 is significantly diminished. The result of the aforementioned conditions in a SiC IGBT device is that holes from the injector region 20 do not adequately modulate the conductivity of the portion of the drift region 22 above a certain distance from the injector region 20. Accordingly, electrons from the emitter contact 18 are met with a high-resistance path in the upper portion of the drift region 22, thereby increasing the on resistance RON of the conventional IGBT device 10 significantly, or cutting off current flow in the device altogether.
In addition to the shortcomings discussed above, the conventional IGBT device 10 is only capable of uni-directional conduction, from the emitter contact 18 to the collector contact 14. Specifically, the first junction J1 in the conventional IGBT device 10 generally prevents current from flowing from the collector contact 14 to the emitter contact 18. Accordingly, the conventional IGBT device 10 is not suitable for switching applications requiring reverse conduction capability. In order to use the conventional IGBT device 10 in applications requiring reverse conduction capability, an external anti-parallel diode must be placed between the collector contact 14 and the emitter contact 18. Integrating the conventional IGBT device 10 with an external anti-parallel diode in this manner allows the conventional IGBT device 10 to conduct in both directions. Although generally effective, the external anti-parallel diode adds cost and area to the resulting bi-directional conducting device.
Accordingly, an IGBT device is needed that is capable of taking advantage of the performance improvements inherent to wide band-gap semiconductor materials, while simultaneously being capable of bi-directional conduction.